A photovoltaic cell, commonly termed a solar cell is an example energy harvesting transducer by which energy is captured from the environment and stored or used to feed other circuits, usually termed a load. Kinetic, thermal, solar, biochemical and electromagnetic energy can be transformed into electricity using appropriate harvesting transducers. A GaAs photovoltaic cell can produce a small amount of power (typically in a range from 0 watt (W)) to 1 W at low-voltage (typically in a range from 0 volts (V) to 2.4V direct current (DC)).
Spacecraft solar power subsystems use solar cells to convert solar energy in order to charge onboard batteries and to power onboard loads. In typical spacecraft power subsystems, a number of solar cells are mounted onto a carrier to form a solar panel. A number of cells on a panel are connected in series to provide a desired output voltage with a number of strings connected in parallel to increase the power from a panel. If there is shadowing on portions of the series string, or if there is a failure of one or more solar cells within the string, then that string cannot contribute power and the peak power available from the panel will be reduced. The reduction of panel peak power in this instance would be due to the output of the shadowed string until all other strings fall to the potential of the shadowed string. Also, if multiple solar panels are placed in different orientations on the spacecraft to accommodate various orientations relative to the sun, these panels cannot be continuous strings of solar cells. In other words, if multiple solar panels are mounted on different faces of a spacecraft, a string cannot start on one face and then finish on another face. Shadowing of the portion of the string which is mounted on the non-illuminated face would reduce output from the entire string. Furthermore, cells are usually arranged in a rectangular configuration of blocks interconnected with wire harnesses. These constraints limit the optimal placement of solar cells and reduce the number of cells which can be placed on irregular geometries.
In order to drive an appropriate spacecraft load circuit from the solar panel, a power converter may be implemented at the output of the solar panel to provide requisite power to the load and to charge the spacecraft batteries. For example, in spacecraft applications, a spacecraft power bus is nominally 28V DC but in practice provides a variable impedance load and is permitted to vary from 22V to 34V DC.
Harsh operating environments, such as, spacecraft environments, military environments, extreme weather environments and inaccessible environments, place constraints on power converter circuit design. For example, the energy harvesting transducers provide low output voltage and low output power. Typical power converters contribute their own losses in terms of power conversion and thus require many transducers to be connected in series in order to provide a reasonable output voltage and power. For a solar power system used in a spacecraft environment, the photovoltaic cells experience extremes of thermal swings, ranging from −150° C. to +120° C. within minutes, and high levels of radiation present in space. Power converters are usually placed inside the spacecraft structure remote from the solar cell array, so that the effect of temperature variation and radiation is much smaller. However, this contributes further to inefficiency due to voltage drops in the wiring harness between the solar cell array and its power converter. Furthermore, in these environments, reliability is important and component failure rate must be minimal.